This invention relates to bistable electromechanical actuators such as those which may be used with intake and exhaust valves for the combustion chambers of an internal combustion engine. Such valves are customarily mechanically activated by a camshaft; but several actuators using electromagnetic forces have been suggested in the prior art. The latter actuators, if practical, have potential for improving engine performance by providing control of intake and/or exhaust valve operation and thus making valve timing variable in engine operation; however, none suggested so far has been sufficiently practical to supplant the ordinary mechanical actuating schemes.
One type of electromagnetic valve actuating device which has been suggested is the solenoid. Conventional solenoids operate by electromagnetically generated attractive forces built by inducing a flux in a moving armature. The magnitude of these forces, however, decreases rapidly over the distance which the armature travels. In typical engine valve applications, this is equal to the valve lift of typically 10 mm, a comparatively large distance. One proposed solution is the helenoid actuator suggested by A. H. Seilly in the SAE Paper No. 790119 entitled "Helenoid Actuators--A New Concept in Extremely Fast Acting Solenoids", published in 1979. In a helenoid actuator, a plunger is moved over a smaller gap with the displacement being amplified by a lever. The lever, however, adds mass to the system; and a large amount of energy is required to move the valve.
A magnet in the armature can help generate strong repulsion forces at the beginning of the armature motion, as shown in the U.S. Pat. Nos. to Kramer 3,202,886, issued Aug. 24, 1965, Stanwell 3,504,315, issued Mar. 31, 1970 and Patel 4,533,890, issued Aug. 6, 1985. However, the solenoid current level of these schemes is high, since it must generate sufficient force to overcome the magnetic attraction as well as to provide the kinetic energy of valve motion. In addition, due to the high seating velocity, braking means may be required for the valve.
Oscillating systems of the spring-mass type, for example, can store amounts of energy significantly larger than the small amount of energy required to overcome friction and spring losses. Solenoids can be used to latch such a system at either end of its stroke. In addition, magnetic forces from the solenoid can compensate for the losses of the system. Such a system is shown in the U.S. Pat. No. 4,455,543 to Pischinger et al, issued June 19, 1984. However, in the system of that patent, electrical energy is continuously consumed while the valve is latched in either of the open and closed positions. In addition, upon system initiation, provision must be made to preload the system by moving the valve against the springs to the open or closed position from a middle position to which it returns when neither solenoid is actuated. A third coil is proposed; and this adds to the complexity of the device. An alternative initialization method not requiring the third coil is proposed in the U.S. Pat. No. 4,614,170 to Pischinger et al. However, this method requires complex control routines and delays start up. In addition, since the valves are open in an intermediate position when the engine is off, Pishinger et al add an auxiliary valve (30 in FIG. 5), which further increases cost and complexity.